1. Field of the Invention
The present invention relates to a glass bending furnace, i.e. a furnace for heating glass sheets to be bent, for the bending of glass sheets to complex shapes, wherein precise control of the temperature profile across each glass sheet is required. The invention also relates to a method of bending glass sheets employing such a furnace.
The shape to which a glass sheet is formed in any bending process is greatly influenced by the temperature of the sheet, for the viscosity of the glass changes rapidly with temperature. Moreover, temperature differences within the glass sheet will similarly have a significant effect. It is therefore highly desirable to precisely control the temperature profile across the sheet in order to successfully produce particular shapes, and also to ensure reproducibility in mass production.
2. Description of the Related Art
It is known from EP 504 117 to control the temperature profile across a glass sheet in combination with a quenching step, so as to differentially toughen the sheet.
A simple method of affecting the temperature profile in a glass sheet to be bent is to place shields over those parts of the glass sheet which are to be heated to a lesser degree, thereby shading those parts, i.e. blocking the heat from reaching them. In service, these shields become hot and themselves become secondary radiators of heat, reducing their effectiveness. U.S. Pat. No. 4,687,501 in the name of PPG Industries Inc seeks to alleviate this problem in the context of gravity bending by provision of secondary shields to shade the glass from the hot primary shields. This is inevitably somewhat cumbersome, and such shields would obstruct the automatic glass handling equipment commonly used in modem factories to transfer the glass sheets from one stage of the process to the next.
Such shields are normally attached to a mould or other support which carries the glass sheet, so that the shields travel with the glass sheet through the furnace, or even the whole bending system. However, a typical glass bending system of the kind in which the glass is heated in position on a mould contains many moulds, each of which has to be equipped with the shields, so a large number of shields is required in total, and any adjustment of the shields required on one mould may similarly be required on every other mould. Since these shields can only be adjusted when a mould is outside the furnace, the shading pattern created cannot change during the passage of a glass sheet through the various heating sections in the furnace. This inflexibility is now seen as a shortcoming, for in the production of the technically increasingly difficult glass shapes demanded by present-day vehicle designers, it is desirable to be able to tailor the heating profile in different heating sections for different purposes. For instance, one may wish to provide extra heating at the corners of the glass sheet, but only when the sheet as a whole has reached the bending temperature. By their very nature, shields of this type are only suitable for reducing the degree of heat the glass sheet receives, and then only in a relatively small area. No attempt is made to direct the heat blocked by these shields to a portion of the sheet in which increased heating is desired.
Furthermore, as normally the shields also remain in position during the passage of the glass through the annealing sections of the bending system, the shields affect the cooling of the glass. This can give rise to undesirable stress patterns in the glass and problems of optical distortion.
It is also known to use heat sinks, i.e. bodies placed near the glass which absorb heat from it, to control the temperature profile in a glass sheet. However, the effectiveness of heat sinks also reduces as they reach the ambient temperature, and as they are also normally attached to the mould or other glass support, their use is accompanied by most of the other disadvantages of shields as well.
GB 2,201,670A proposes the reverse technique to using a heat sink, namely, using a body of thermally insulating material as a heat reflector; a laboratory experiment is described in which such a body is placed beneath a portion of a glass sheet which it is desired to preferentially heat. In a production furnace, such reflectors would also have to be mounted on the moulds (or at least in boxes, or on trolleys, that carried the moulds) and so would be subject to the same problems as described for shields and heat sinks above.
Where an increased heat input is required in a localised area it has long been known to apply extra heat in that particular area by means of auxiliary heaters, known to those skilled in the art as "crease heaters". UK 836,560 is one of many specifications which describe such heaters; in this embodiment the heaters are suspended through slots in the roof of the furnace, but other forms of support are possible. If it is necessary to limit the area of a glass sheet heated by the crease heater, adjacent parts of the glass sheet may be shaded from it as is shown in FIG. 10 of EP 338 216 A2.
Although such crease heaters serve a useful purpose, they also have many disadvantages, such as obstructing the space in the furnace above the glass sheets, and being prone to damage or misalignment. They cannot be used over the centre of the sheet, where a long support arm would be needed which would itself shield the glass. Because of the need for support, crease heaters cannot be made large enough to cover substantial areas. Attempts to alleviate some of the problems, e.g. automating the adjustment and insertion/removal of crease heaters with servo motors, entails great expense and the risk of unreliability. Furthermore they are not suitable for creating non-local temperature differentials, such as a centre-edge differential controlled over the entire centre-edge distance in the glass sheet.
One way of controlling the temperature profile across the whole glass sheet is to arrange separate regulation of separate areas of heaters, or indeed of separate heating elements, in the glass bending furnace. For example, EP 443 948 A1 discloses a furnace which includes sets of electrical resistances in its upper part, with the temperature or power of the sets independently regulated. The orientation and location of such sets of resistances are also arranged to optimise control of the temperature profile in a glass sheet. In such a furnace, it is possible to control the temperature profile across the whole glass sheet. The object may be to obtain as uniform a temperature as possible in the sheet, or to create a particular centre-edge temperature differential, according to the requirements of the particular shape to be formed.
However, there are limitations on the magnitude of temperature differentials thus produced. If a particular set of elements is run at high power to preferentially heat the part of the glass sheet directly below that set, adjacent parts of the sheet will inevitably also receive extra heat. This may be controlled to a certain extent by reducing the distance from the elements to the glass sheet. While reduction of this distance will reduce unwanted heating of adjacent parts of the glass sheet, it can result in optical distortion of the glass sheet if this distance becomes too small. Hence there remains a need for a further technique.
In EP 443 948 A1, further control of the temperature profile is obtained by providing additional heating elements in the walls of the furnace. For example, the last paragraph of the description describes the manufacture of an S-shaped glazing, and the need for a marked temperature difference between certain parts of the glass sheet. The last sentence of this paragraph explains that it is advantageous to heat the upwardly convex part of the S-shaped glazing by the wall heaters to avoid overheating the central part of the sheet, but of course, the provision of extra heaters in the walls of the furnace entails extra expense; it would be desirable to obtain the further temperature control required for certain products using heating elements in the roof only.
It is easier to control the bending of glass in a so-called "simple" bend, i.e. when the axes of curvature are parallel or only at small angles to each other, and this has usually been the case in the past. However, glass of complex shape, that is, glass having curvature in two substantially perpendicular directions, is increasingly being required, e.g. for automotive applications, and this presents more difficulties.
When such glass is produced by means of the gravity or "sag" bending process, difficulty has for example been experienced in obtaining the desired cross curvature profile. By cross curvature profile is meant change in curvature in a direction extending from the top to the bottom of the window, e.g. a windscreen, as seen in its installed condition. Such curvature is about one or more substantially horizontal axes extending from one side of the vehicle to the other. Often a uniformly circular cross curvature profile is desired, but in practice a flatter region is obtained in the central region of the windscreen with most curvature near the top and bottom. This can result in the perception of an unacceptable secondary image by the driver. In cases where an increased degree of complex curvature is required, and/or the height of the windscreen increases relative to its width, an inverse cross curvature may occur in the centre of the windscreen, so that a cross-section on the axis of symmetry would begin to resemble an inverted letter "w". In addition to optical problems, this results in poor wiper performance.